IR Handout

Absorption Spectroscopy

Type of Radiation

Energy (kcal/mol)

Frequency

Molecular Interaction

Wavelength

Translation

Microwave

0.001 - 1

1 - 400 cm-1

Molecular Rotations Rotation of the molecular as a whole about its center of mass

400 - 4,000 cm-1

Molecular Vibrations Stretching and binding of bonds

Infrared

1.1 - 11

Visible

50 - 75

400 - 600 nm

Ultraviolet

75 - 150

200 - 400 nm

Electronic Excitation Promotion of electrons to higher energy levels

Infrared Absorption Spectroscopy 4000 cm-1

2000 cm-1

X-H

Region 2500 - 4000 cm-1

C-H N-H O-H Stretching

SP

X=Y

Region

Region

2000 ­ 2500 cm-1

1500 2000 cm-1

C=C C=N Stretching

400 cm-1

1500 cm-1

C=C C=O C=N Stretching

Fingerprint

Region

400 - 1500 cm-1

Single Rond Stretching Bond Bending Polyatomic Vibrations

Figure by MIT OCW.

Infrared Spectra: Tables of Reference X-H Region Phenols and Alcohols

ROH

3700-3500 sharp or 3200-3600 broad(H-bonded)

Acids

RCO2H

2800-3600 very broad

Amides and Amines

RCONHR R2NH

3300-3500

C-H bonds

C C H C C H C C H RCHO

3100-3300 3000-3200 2850-3000 2700-2800

sp Region Acetylenes Nitriles Ketenes Allenes

C C C C

C N C O C C

2100 2200 2150 1950

Double Bond Region Alkenes Imines Nitro

C C C N NO2

1600-1670 weak unless conjugated 1600-1700 1350-1550(two bands)

Carbonyl Groups Note: subtract ca. 30 cm-1 for conjugation (e.g. with a double bond or aromatic ring) Anhydrides RC(O)OCOR Acid Chlorides RCOCI Esters Acids

1710

RCO2H

1700-1725

1740

O

1715 1780

1740 O

NR

O O

O O

1770

O

1700 1720-1740

1680

O

O

1630-1700

O

O

1790-1815 1725-1755

Urethanes R2NCO2R Aldehydes RCHO

O 6-membered and larger cyclic ketones

1740-1780, 1800-1840 (two bands)

RCO2R

Amides RCONR2

Ketones (subtract ca. 30 cm-1 for R2C O 1710 conjugation)

1690-1740

O

1730

NR

1650

Figure by MIT OCW.

Regions of the 13C NMR Spectrum

Aromatic 120-160 ppm

Carbonyls 150-220 ppm 200

180

C-O, N 40-90 ppm

Alkenyl 100-150 ppm 160

140

120

Alkynyl 70-90 ppm 100

80

Alkyl 0-50 ppm 60

40

20

0

Chemical Shift (�)

Figure by MIT OCW. Characteristic Functional Group Chemical Shifts in 13C NMR (ppm) Alkanes Methyl (RCH3) Methylene (RCH2R’) Methine (RCH(R’)(R”)) Quaternary (RC(R’)(R”)(R’’’)) Alkenes Aromatic Alkynes Nitriles Alcohols, Ethers Amines

0-30 15-55 25-55 30-40 100-150 120-160 70-90 110-125 50-90 40-60

Organohalogen C–F C–Cl C–Br C–I Ketones, Aldehydes Carboxyl Derivatives Acids Esters Amides Carbamates

70-80 25-50 10-40 –20-10 185-220 150-185 155-180 150-180 150-160

1

H NMR Spectra: Tables of Reference

Average Chemical Shifts (�) of �−Hydrogens in Substituted Alkanes* CH3X

X

RCH2X

R2CHX

0.9 1.25 4.4 3.4 3.3

1.25 1.5

F Cl Br

0.233 0.9 4.26 3.05 2.68

I OH OR OAr OCOR

2.16 3.47 3.3 3.7 3.6

3.2 3.6 3.4 3.9 4.1

4.2 3.6

OCORAr SH SR SOR SO2R

3.8 2.44 2.1 2.5

4.2 2.7 2.5

5.1

2.8

2.9

3.1

NR2

2.2 2.9

2.6

2.9

H CH3 or CH2

NR-Ar NCOR NO2 CHO COR COAr COOH COOR CONH2

1

CR=CRCR Phenyl Aryl � �

C CR C CN

2.8 4.28 2.20 2.1 2.6 2.07 2.1 2.02 2.0-1.6 2.3 3.0-2.5 2.0 2.0

4.0 4.1

5.0

2.8

4.4 2.3 2.4 3.0 2.3 2.3 2.2 2.3 2.7

3.2 4.7 2.4 2.5 3.4 2.6 2.6

Chemical Shifts of Hydrogens Bonded to

Unsaturated Centers Unconjugated

Conjugated*

R2C=CH2

4.6-5.0

5.4-7.0

R2C=CHR

5.0-5.7

5.7-7.3

Aromatic

6.5-8.3

Nonbenzenoid aromatic

6.2-9.0

Acetylenic

2.3-2.7

2.7-3.2

Aldehydic

9.8-9.8

9.5-10.1

R2NCHO

7.9-8.1

ROCHO

8.0-8.2

Type

* The position depends on the type of functional group in conjugation with the unsaturated group.

Chemical Shifts of Hydrogen Bonded to Oxygen, Nitrogen, and Sulfer Functional Group OH

2.6 2.9 NH2

2.3

2.7

Chemical Shift, �

Alcohols

0.5 0.5-5

(Monomeric) (Associated)

Phenols

(Monomeric) (Associated)

Enols

4.5 4.5-8 15.5

RCO2H

9-12

(Dimeric)

H-bonded to C=O

13-16

Alkylamine

0.6-1.6

Arylamine

2.7-4.0

Amide

7.8

Alkylamine,

0.3-0.5

Arylamine

2.7-2.8

* The tabulated values are average values for compounds that do not contain another functional group wlthin two carbon atoms from the indicated hydrogens.

NH R3NH+

Ammonium salts

7.1-7.7 (in CF3COOH)

Includes polycyclic and many heterocyclic aromatics.

SH

Aliphatic Aromatic

1.3-1.7 2.5-4

�

Figure by MIT OCW.

�

1H NMR Coupling Constants (Expanded)

H

geminal 12-15 Hz acyclic can be 0-25 Hz in cyclic system

H

H

6-8 Hz vicinal (averaged by free rotation)

H

In rigid systems, vicinal coupling can range from 0 to 15 Hz. For example: H

H

ax-ax

H

H

6-14 Hz

H

ax-eq

0-5 Hz

H

H

H

eq-eq 0-5 Hz

Spin-spin coupling in alkenes: H geminal

0-3-Hz

H H

H

trans

12-18 Hz

cis

H

6-12 Hz

allylic 0-3 Hz H

Spin- spin coupling in arenes: Ha Hb Hc Hd

Jab (ortho)

6-10 Hz

Jac (meta)

1-3 Hz

Jad (para)

0-1 Hz

Note: Structures shown above represent generic coupling situations and not the specific molecules depicted (in which the labeled protons would be chemically equivalent and would not couple). Figure by MIT OCW.